staging: lustre: llite: ensure all data flush out when umount

Write out all extents when clear inode. Otherwise we
may lose data while umount.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5584
Reviewed-on: http://review.whamcloud.com/12103


Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>